1. Field
Described herein is a device for feeding sheets one by one from a pile or stack of sheets to a transportation device for transporting the sheet to a process station, the device comprising a low-pressure chamber, a number of separately driven shafts which are positioned perpendicular to the direction of transportation and are arranged in the low-pressure chamber essentially equidistantly spaced from one another and which each carry a plurality of wheels with friction lining, each shaft being driven by its own motor which is connected to and controlled by a control unit, and a separating device which is arranged essentially vertically above the low-pressure chamber and at a distance from the low-pressure chamber that is somewhat larger than the thickness of a sheet.
Described herein is also a method for feeding sheets one by one from a stack of sheets to a transportation device for transporting the sheet to a process station.
The method and a device described herein are particularly suitable for, but not limited to feeding cardboard blanks, for instance corrugated cardboard, from a stack of blanks to a machine for applying text and/or symbols or for punching.
The problems which arise when feeding a (lowermost) sheet of a stack can be explained by the fact that, in practice, it is extremely difficult to feed a sheet without a certain degree of skidding between feeding wheels and sheet, which causes poor repeatability. This is due to the fact that the friction between wheel and sheet changes with the continuously changing number of sheets in the sheaf, type of sheet (surface structure, thickness/weight etc.), changes in speed etc. In conventional she et feeding devices, this has partly been solved by using feeding rolls. A major disadvantage thereof is that sheets of corrugated cardboard are easily deformed or crushed in the press roll nip, which has a detrimental effect on the stackability, shape permanence, etc of the box subsequently produced. In order to minimize the sliding between wheels and the sheet being fed, a large vacuum (negative pressure) has to be used. However, this implies that the next sheet is put down too fast and the contact force between the retarding feeding wheels will be strong, which damages the sheets and wears the wheels out. There is also a risk that the next sheet is fed towards the front sheet support or the separating device, which results in damage to the front edge of the sheet.
This can also lead to the sheet feeding being interrupted when jamming occurs, i.e. two sheets (the one to be fed and the sheet on top thereof) are fed simultaneously into the gap between the sheet support and the feeding table and get stuck. Theoretically, this would be counteracted if a motor with a sufficient braking torque could be used. Then it would, theoretically, be possible to retard the wheel shafts in a considerably shorter time or over a considerably shorter distance. However, this is limited by the performance of commercially available motors which have either too low a maximum torque or too high a mass-moment of inertia. In order to counteract the above-mentioned problems, the vacuum has to be decreased, which has a detrimental effect on the repeatability when uncontrollable sliding (which also depends on the speed, the height of the sheaf etc.) appears.
2. Description of Related Art
A sheet-feeding device of the type defined above is already known from U.S. Pat. No. 5,006,042. This known sheet-feeding device comprises a low-pressure chamber having an integrated feeding table on which a stack of sheets is intended to be placed, and a sheet support at a distance above the feeding table in the order of the thickness of one sheet. A number of shafts are arranged in the low-pressure chamber. The shafts carry a plurality of wheels which protrude through openings in the feeding table and serve to transport the lowermost sheet of the stack through the gap between the feeding table and the sheet support to a belt conveyor. Each shaft is driven by a separate motor. With reference to the reasoning above and to the fact that the distance is relatively large between the wheel shaft closest to the sheet support and on the one hand the sheet support and, on the other hand, the belt conveyor, there is an imminent risk that the sheets arrive inclined and/or with so-called index deviation at the belt conveyor with ensuing problems in the subsequent process station(s). No correction for the above-mentioned deficiencies is indicated in said patent specification. Furthermore, waiting sheets in the stack or sheaf, which due to frictional forces are pressed towards the sheet support (especially at a high level of vacuum), tend to get stuck with their front edge on the sheet support and, thus, be prevented from being correctly put down when sheets that are being fed have completed their feeding cycle. Often a corner of the front edge is pressed against the sheet support. Once the feeding cycle starts, the sheet is damaged or stuck on the sheet support and is not fed in a correct way.
Other problems that are related to sheet-feeding devices of the above-mentioned type are, for example, the following ones: If a “normal” so-called cam graph (movement pattern) in the sheet-feeding cycle is used (see FIG. 7a of U.S. Pat. No. 6,543,760), when changing the speed, the acceleration and retardation ramps (the inclination of the graphs) will change. This implies that, at decreased machine speed, lower retardation of the feeding wheels and longer time to stop the wheels are obtained, although a force for bringing about a faster stop is available in the motor. Consequently, there will be enough time for the next sheet of the sheaf to be sucked down onto the wheels before they have stopped. As a result, the surface layer of the sheet could be damaged by the wheels which spin intensively against the same (“rubbing”) and the sheet is advanced to the front sheet support in an uncontrolled manner. Variations in parameters, such as size of sheet, height of sheaf, level of vacuum and machine speed, also result in a change in the total friction acting between sheet and wheels. The variations in friction give rise to variations in the sliding between sheet and wheels which always occurs in connection with the acceleration of a sheet. When the sliding varies, it appears as variations in the index of the sheet. Moreover, there is the ubiquitous stochastic variations in friction from one sheet to another depending on, inter alia, the individual surface structure of each sheet, turbulence in vacuum boxes (low-pressure chambers) etc. which give a stochastic index adding to the above-mentioned reasons for inadequate repeatability.
The starting material for production when using so-called inline machines is corrugated cardboard with formats adapted to the respective series of boxes to be produced. The feeding accuracy is decisive for the positioning of the printing image, slits and punch holes relatively to the front end and the rear end, respectively of the sheet. Accurate positioning of the printing image, slits and punch holes and excellent repeatability from one sheet to the next is essential for the quality of the boxes produced in the converting machine, for example the inline machine. The term feeding accuracy also covers straight feeding relatively to the front and rear end of the sheets. This is a prerequisite for the accuracy in the geometry of the boxes produced and, thus, in the folding process of an inline machine.
Modern converting machines adapted for corrugated cardboard, in particular inline machines, are characterised by high productive capacity. In this connection, high speed is a decisive factor.
So far, attempts to optimise the combination of related properties, feeding that does not crush the sheets, adequate repeatability and high speed, have only been partly successful. It has turned out to be difficult to develop a feeding that is optimised in all areas. Either feeding rolls are used, by means of which a relatively acceptable result is obtained with regard to feeding accuracy and speed, or a system is used which operates without feeding rolls, in which case acceptable accuracy is obtained only at limited speeds. U.S. Pat. No. 6,543,760 discloses a feeding system that is said to be characterised by a combination of the above-related properties. However, it has been found difficult to achieve this combination of high performance, feeding accuracy in connection with said feeding without feeding rolls. The direct cause for this is related to the fact that it has been found that the feeding wheels of this table cannot be stopped as rapidly as required. This is a problem in particular at high speeds, because of the physical properties of the system in combination with the performance of the servo systems available today. It has been found to be impossible to avoid the undesirable roll out of the feeding wheels (or stopping distance). The roll out has a direct affect on the possibility of operating the unit at higher speeds with unchanged feeding accuracy.
U.S. Pat. No. 5,048,812 discloses a sheet feeding device without feeding rolls for feeding of sheets one by one to a process station or sheet processing machine. The device consists of a vacuum box on the top portion of which the sheets are fed and a gate or separating device which releases only one sheet at a time from a stack of sheets to said machine. The vacuum box comprises a first and a second motor-operated drive gear, the first gear, which is located underneath the stack of sheets, being operated at a variable speed while the second gear is operated at a constant speed. Each gear drives a number of shafts at the same speed of rotation and feeding wheels for feeding sheets are arranged on said shafts. Adjacent the vacuum box a housing is provided which contains a motor-driven shaft on which a number of cams are attached. From the vacuum box and directly below and parallel to the wheel shafts underneath the stack of sheets, an associated cam shaft extends into the housing and each cam shaft is provided, inside the housing, with a cam follower engaging the associated cam. Each camshaft is pivotally journalled in the vacuum box and there carries a number of raising elements, which can raise a corresponding number of support elements. These support elements are displaceably positioned around each wheel shaft and between each wheel on the shaft. Programming and adjustment, respectively, of the raising cycles is not possible because of said mechanical, motion-transferring mechanism (cams and cam follower). The support elements can be inactivated only by locking their respective cam followers.
The feeding cycle according to U.S. Pat. No. 5,048,812 starts by the support elements, on which the stack of sheets rests, being lowered from their initial raised positions, so that the lowermost sheet of the stack is brought in contact with the non-rotating feeding wheels, which are subsequently caused to rotate. When the front edge of the sheet being fed hits the feeding wheels of a shaft (27) between the gate and the delivery side (42) the support elements (at 21) closest to the feeding side (38) are raised. The front edge of the sheet then hits the feeding wheels of the next shaft (29) and the succeeding support element (at 23) is raised, and so on until all the support elements are raised and carry the stack of sheets.
In brief, all the wheels underneath the stack of sheets rotate during the whole feeding cycle and at the same speed of rotation. The support elements are raised purely mechanically following a sequence and remain raised until the next sheet feeding cycle begins. Moreover, the support elements and their respective raising mechanisms have a large mass, which reduces the speed and precision of the raising cycles. (Re)programming of the raising cycles is not possible, nor is it possible to drive (or stop) the feeding wheels of a drive shaft at another speed of rotation than that of the feeding wheels of an adjacent drive shaft.